Efforts have been made to develop a closed wafer cassette (hereinafter referred to simply as "closed cassette") intended to avoid the contamination of semiconductor wafers (hereinafter referred to simply as "wafers") with particles (Monthly Semiconductor World, January, 1997). As shown in FIG. 10, this closed cassette 1 has a cassette body 10 for containing, for example, thirteen wafers W, and a lid 11 capable of hermetically closing an opening formed in one side of the cassette body 10. The lid 11 can be fastened to the cassette body 10 by means of a key, not shown.
A method of using this closed cassette 1 will be described as used in combination with a heat treatment apparatus. A working zone S1 in which the closed cassette 1 is carried by an operator or a carrying robot, and a loading zone S2 of a cleanliness higher than that of the working zone S1 are separated from each other by a wall 13. The closed cassette 1 is joined to a surface of the wall 13 on the side of the working zone S1 so as to correspond to a gateway 14 formed in the wall 13. In FIG. 10, indicated at 15 is a cassette table. Normally, the gateway 14 is closed by a door 16. The lid 11 is opened by an opening device 17 built on the door 16.
After removing the lid 11 from the cassette body 10, the lid 11 is carried into the loading zone S2 together with the door 16 by a door operating mechanism 18 which moves the door 16 vertically and horizontally. Then, the wafers W are taken out of the cassette 1 by a transfer mechanism, not shown, disposed in the loading zone S2.
In the loading zone S2, the wafers W are transferred to a wafer boat, not shown, and the wafer boat carrying the wafers W are carried into a thermal treatment furnace to subject the wafers W to a predetermined thermal process. The wafers W processed by the thermal process are returned into the closed cassette 1, the closed cassette 1 is conveyed to the next processing station, such as a cleaning station. Then, the lid 11 is removed, the wafers W are taken out of the cassette 1 and are subjected to a cleaning process.
In this thermal treatment apparatus, the loading zone S2 is kept at a positive pressure higher than that in the working zone S1 so that clean air flows from the loading zone S2 into the working zone S1 to prevent particles from entering the loading zone S2. The positive pressure in the loading zone S2 is higher than the atmospheric pressure by, for example, 1 Pa or above, preferably, by about 500 Pa.
The interior of the closed cassette 1 is purged by clean air or nitrogen (N.sub.2) gas. The pressure in the cassette 1 is approximately equal to 1 atm, the atmospheric pressure. Since the pressure in the closed cassette 1 is slightly lower than that in the loading zone S2, the lid 11 is difficult to open when an effort is made to open the lid 11 after joining the closed cassette 1 to the wall 13 with the opening of the closed cassette 1 coincided with the gateway 14, and large power is necessary to drive the door operating mechanism 18 for removing the lid 11 from the cassette body 10.
The temperature of the wafers W immediately after the completion of the thermal process is, for example, on the order of 100.degree. C. If the wafers W heated at about 100.degree. C. are put in the closed cassette 1 and the wafers Ware cooled to a room temperature, the pressure in the closed cassette 1 drops to a pressure slightly below the atmospheric pressure. Accordingly, large power is necessary for removing the lid 11 from the cassette body 10 when opening the cooled closed cassette 1 at the cleaning station to take out the wafers W from the closed cassette 1 because the cleaning station is kept at the atmospheric pressure.
If the pressure in the closed cassette 1 is lower than the pressure in an atmosphere in which the closed cassette 1 is to be opened, the gas prevailing in the atmosphere will blow suddenly into the cassette 1 upon the removal of the lid 11 from the cassette body 10, blowing up particles.